Property of DC breakdown in XLPE containing cross-linking by-products

XLPE (Cross-Linked Polyethylene) cable is expected to be applied to a long distance power transmission in HVDC systems. DC transmission system is effective for a long distance power transmission, and XLPE cable is not necessary to give a periodical maintenance procedure. However, when a DC high voltage is applied to XLPE, it is said that an electrical breakdown may occur due to an effect of space charge accumulation. In the previous research, the space charge distribution and an external circuit current in the XLPE including the by-products were observed simultaneously in a same sample using PEA (Pulsed Electro-Acoustic) measurement system under a DC high electric field. Although many dielectric breakdowns were observed in this investigation, they occurred in the state when the enhanced field by the accumulated space charge was relatively lower than that observed in LDPE. On the other hand, a large conduction current was observed in the XLPE sample, and it was predicted that a thermal breakdown may occurred due to an energization by the large current flowing in the XLPE at the breakdown. Therefore, in this paper, we tried to understand the relationship between the dielectric breakdown and the time dependence of the energy consumption distribution. Judging from the time dependence, it was found that the increase rate of the average consumption energy in bulk of XLPE was larger, the time to breakdown was shorter. By observation of the dissipation power distribution, it was also found that the large dissipation power zone was constantly localized in the part of the bulk in XLPE at the breakdown. Judging from the results, we considered that the process until dielectric breakdown occurred as follows; at first, a rapid heat increase generates intensively in a part of the bulk of the sample in the thickness direction, then, at a weak point in the layer, a melting happens and finally the dielectric breakdown occurs at the point.